Fragmentation point and simulation site adjustment for resolution enhancement techniques

ABSTRACT

A method of performing a resolution enhancement technique such as OPC on an initial layout description involves fragmenting a polygon that represents a feature to be created into a number of edge fragments. One or more of the edge fragments is assigned an initial simulation site at which the image intensity is calculated. Upon calculation of the image intensity, the position and/or number of initial simulation sites is varied. New calculations are made of the image intensity with the revised placement or number of simulation sites in order to calculate an OPC correction for the edge fragment. In other embodiments, fragmentation of a polygon is adjusted based on the image intensities calculated at the simulation sites. In one embodiment, the image intensity gradient vector calculated at the initial simulation sites is used to adjust the simulation sites and/or fragmentation of the polygon.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of U.S. Provisional PatentApplication Nos. 60/564,138, filed Apr. 21, 2004, titled METHOD FORDYNAMICALLY ADJUSTING SITES FOR USE WITH OPC USING GRADIENTS OF AERIALIMAGE, and 60/547,484, filed Feb. 25, 2004, titled CONCEPTS IN OPTICALAND PROCESS CORRECTION, which are herein incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to photolithographic processing ingeneral, and in particular to layout correction for resolutionenhancement techniques such as optical and process correction (OPC).

BACKGROUND OF THE INVENTION

In the conventional photolithographic processing of integrated circuits,features are created on a semiconductor wafer by exposing the wafer withlight or radiation that is passed through a mask or reticle. A typicalmask/reticle has patterns of opaque and clear areas that selectivelyexpose corresponding areas of light-sensitive chemicals on the wafer.The exposed areas are chemically and mechanically processed to createthe desired features on the wafer.

As the size of features being created on a wafer approaches and becomessmaller than the wavelength of radiation used to expose the wafer,optical distortions can occur whereby the pattern defined on the mask orreticle will not match the pattern of features that are created on thewafer. To improve the pattern fidelity, changes can be made to themask/reticle patterns that compensate for the expected opticaldistortions. One common tool for adjusting the mask/reticle pattern isan optical and process correction (OPC) tool such as the CALIBRE®software tools available from Mentor Graphics Corporation, the assigneeof the present invention.

As will be appreciated by those skilled in the art, an OPC tool works toproduce a corrected mask/reticle by reading at least a portion of alayout design that is defined in a database. Each feature to be createdon the wafer is defined as a series of vertices that make up a polygonhaving a shape of the desired feature. The polygons are fragmented bydividing the perimeter of the polygon into a plurality of edgefragments. An edge placement error (EPE) is computed for each edgefragment that compares where an edge fragment will be printed on a waferversus its desired position. The OPC tool then moves the edge fragmentsin order to precompensate for the expected optical distortions that willoccur during processing so that the position of the edges created on awafer will more closely match the desired positions.

FIG. 1A illustrates a representative polygon 10 that defines arectangular feature to be created on a wafer. In order to correct foroptical distortions, the polygon 10 is divided into a plurality of edgefragments that are bounded by fragmentation end points 12. During OPC,at least some of the edge fragments positioned between the fragmentationend points 12 are moved inwardly or outwardly to compensate for opticaldistortions. In the example shown in FIG. 1A, the polygon 10 does notcontain a sufficient number of fragmentation points 12 to create therectangular feature on the wafer with an acceptable image fidelity. Asimulated aerial image 14 plots where the edge fragments will be printedon a wafer. In the example shown, the fragmentation of the polygon 10 istoo coarse in order to be able to finely correct for the opticaldistortions that may occur during processing. Conversely, FIG. 1Cillustrates a polygon 10 including more than enough fragmentation endpoints 12 to finely adjust for the optical distortions that may occurduring processing. Although the number of fragmentation end points 12 issufficient in the example shown in FIG. 1C, the time required to computethe OPC corrections of each individual edge fragment may be prohibitive.Therefore, it is desirable to divide the polygon 10 in a manner as shownin FIG. 1B with a sufficient number of fragmentation end points 12 sothat image fidelity is acceptable and processing time is notprohibitive.

Associated with each edge fragment is a simulation site that defines anumber of sample points at which the image intensity duringphotolithographic processing is estimated. From the estimated imageintensity points, a determination is made of the expected edge placementerror (EPE) of the edge fragment. FIG. 2 shows a conventional,simplistic method of placing the simulation sites on the edge fragments.Simulation sites 16 a are placed in the center of the edge fragmentsthat are at the ends of the polygon and simulation sites 16 b arepositioned at the location of the fragmentation end points 12 that areadjacent to the corners of the polygon. Additional simulation sites 16 care placed in the center of the edge fragments that are between thefragmentation end points 12 for the remainder of the polygon. Comparingthe location of the simulation sites with simulated aerial image 18(which is a plot of the estimated image intensity at a value that willexpose the chemicals on the wafer), it can be seen that many simulationsites are not positioned at the place where the aerial image intensitydeviates most significantly from the desired outline of the polygon 10.Therefore, if OPC corrections are made based on the location of thesimulation sites as originally positioned, the most optimum edgecorrection will likely not be achieved.

To achieve improved OPC corrections, it is desirable to place thesimulation sites and/or use varying numbers of simulation sites atpositions closer to where the EPE of an edge fragment is greatest alongthe length of an edge fragment.

SUMMARY OF THE INVENTION

To improve a resolution enhancement technique such as optical andprocess correction (OPC) of features to be created with aphotolithographic process, the present invention divides layout featuresinto a number of edge fragments. Simulation sites are positioned on oneor more of the edge fragments in order to perform an initial calculationof image intensity. One or more of the simulation sites are then movedto be closer to a point of greater edge placement error (EPE) for anedge fragment. In one embodiment of the invention, one or more of theinitially placed simulation sites are repositioned based on an imageintensity gradient vector angle that is calculated at the simulationsites.

In another embodiment, additional simulation sites are positioned atlocations on an edge fragment where the image intensity gradient vectorindicates a curve in the image intensity along the edge fragment. In yetanother embodiment of the invention, additional sample points are addedto a simulation site where image intensities are calculated. In yetanother embodiment, additional fragmentation end points are added orremoved in accordance with the estimated image intensity gradientvectors. Image intensity calculations or EPEs that are calculated fromthe image intensities at the additional simulation sites or theadditional sample points, are used to determine a desired OPC correctionfor the edge fragments.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same become betterunderstood by reference to the following detailed description, whentaken in conjunction with the accompanying drawings, wherein:

FIGS. 1A-1C illustrate a conventional method of fragmenting a polygoninto a number of edge fragments prior to performing OPC;

FIG. 2 illustrates a conventional method of positioning simulation siteson edge fragments;

FIG. 3A illustrates a fragmented polygon having an initial simulationsite placement in accordance with one embodiment of the presentinvention;

FIG. 3B illustrates a number of image intensity gradient vectors thatare computed at the simulation sites shown in FIG. 3A in accordance withan embodiment of the present invention;

FIG. 3C illustrates a fragmented polygon having a revised simulationsite placement in accordance with one embodiment of the presentinvention;

FIG. 3D illustrates one method of determining where a simulation siteshould be repositioned based on a computed image intensity gradientvector in accordance with one embodiment of the present invention;

FIG. 4A illustrates a fragmented polygon having an initial simulationsite placement in accordance with another embodiment of the invention;

FIG. 4B illustrates a number of image intensity gradient vectors thatare computed at several of the initial simulation site placements;

FIG. 4C illustrates a fragmented polygon having additional simulationsites added to edge fragments in accordance with the computed imageintensity gradient vectors according to another embodiment of thepresent invention;

FIG. 5 illustrates a simulation site having additional sample pointsadded in accordance with another embodiment of the present invention;

FIG. 6 illustrates a grid of sample points at which simulations areperformed for fragmenting a feature in accordance with anotherembodiment of the invention;

FIG. 7 illustrates a grid of sample points with a filter that eliminatessample points that are not in proximity to a feature edge; and

FIG. 8 illustrates one possible computing environment for performing theembodiments of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

To improve the optical and process correction of features to be createdby a photolithographic process, the present invention uses a betterplacement of simulation sites and/or the addition of simulation sites,sample points or fragmentation end points to an edge fragment. Althoughthe invention is primarily used in the creation of integrated circuits,it will be appreciated that the invention could be used with any featureto be created with a photolithographic process includingMicro-Electrical-Mechanical Systems (MEMs), recording heads for diskdrives, etc.

FIG. 3A illustrates a polygon 50 that defines a feature to be created ona wafer with a photolithographic process. The polygon 50 includes anumber of fragmentation end points 52 that divide the perimeter of thepolygon into a number of edge fragments. Associated with one or more ofthe edge fragments are simulation sites 54 a, 54 b, 54 c . . . 54 n, atwhich a number of the image intensity calculations are made. From theimage intensity calculations, a determination can be made of theexpected EPE for the corresponding edge fragment.

In one embodiment of the invention, the placement of one or more of thesimulation sites 54 a-54 n is modified from their initial placement inorder to improve the accuracy of the OPC corrections made to the edgefragments. As shown in FIG. 3B, a number of image intensity gradientvectors 56 are calculated at one or more of the initial simulation sites54. In one embodiment of the invention, the image intensity gradientvectors 56 define the orientation of the image slope at the simulationsite versus the orientation of the edge fragment.

As will be understood by those skilled in the art, each simulation site54 includes a pattern of sample points at which the image intensity issimulated. The points generally form a cross with sample points orientedparallel to the edge fragment and sample points oriented perpendicularto the edge fragment. One method of calculating the image intensitygradient vector 56 is to estimate the image intensity at the samplepoints on either side of a center sample point and in a directionparallel to the edge fragment. Estimates of the image intensity oneither side of the center sample point and in a direction perpendicularto the edge fragment are also made. From these estimates, a pair ofvectors are computed and are mathematically combined in a head to tailfashion to compute the magnitude and direction of the image intensity atthe area of the simulation site. The image intensity gradient vector 56is indicative of the expected curvature of image intensity near the edgefragment.

Once the image intensity gradient vectors have been calculated, theresults may be stored for the corresponding simulation sites with a tagor other identifier. Next, one or more of the simulation sites 54 arerepositioned to be closer to a point of greater image intensitycurvature for the edge fragment. As shown in FIG. 3C, a simulation site54 c is moved to a position 54 c′, a simulation site 54 e is moved to aposition 54 e′, and a simulation site 54 j is moved to position 54 j′,etc. With the simulation sites moved, a more accurate determination canbe made how the edge fragments should be OPC corrected in order toproduce better image fidelity.

FIG. 3D illustrates one example of moving a simulation site inaccordance with the calculated image intensity gradient vector. At asimulation site 60, an image intensity gradient vector 62 is calculatedto be oriented five degrees or less towards an adjacent fragmentationend point 64. Therefore, in one embodiment of the invention, thesimulation site 60 is moved in the direction of the gradient to aposition 60A that is 70% of the way between the original location of thesimulation site 60 and the adjacent fragmentation end point 64.Similarly, at a simulation site 66, an image intensity gradient vector68 is calculated to be oriented at an angle of greater than five degreestowards an adjacent fragmentation end point 64. Therefore, in oneembodiment of the invention, the simulation site 66 is moved in thedirection of the gradient, 100% of the way towards the adjacentfragmentation end point 64.

Although the example described above moves the simulation site in thedirection of the gradient either 70% or 100% of the way towards anadjacent fragmentation end point, it will be appreciated that otherdistances could be used based on other magnitude and/or angle thresholdsof the image intensity gradient vector. For example, if an imageintensity gradient vector had an angle of less than 2°, no movement ofthe simulation site may be performed.

FIGS. 4A-4C illustrate another embodiment of the present invention. Inthis embodiment, a polygon 80 (FIG. 4A) is divided into a series of edgefragments using a number of fragmentation end points 82 that arepositioned around the perimeter of the polygon 80. An initial placementof simulation sites 84 is made at which the image intensity is to becalculated. As shown in FIG. 4B, image intensity gradient vectors 86 arecalculated at one or more of the simulation sites 84. For example, imageintensity gradient vectors 86 a, 86 b, 86 c are calculated at simulationsites 84 a, 84 b, and 84 c respectively. If the image intensity gradientvector exceeds some predefined angle or magnitude, then one or moreadditional simulation sites 84 a 1 and 84 a 2 are added to the edgefragment on either side of the simulation site 84 a (FIG. 4C).Similarly, additional simulation sites 84 b 1, 84 b 2, and 84 c 1, 84 c2 are added adjacent the simulation sites 84 b, 84 c. In one embodiment,the additional simulation sites are positioned on either side of theoriginal simulation site. However, other placements may be used.

As an alternative to adding additional simulation sites to an edgefragment, each simulation site may have additional sample points addedif the image intensity gradient vector exceeds a predefined angle ormagnitude. As shown in FIG. 5, a polygon 100 representing a feature tobe created by a photolithographic process includes a simulation site 102having a number of sample points 102 a, 102 b, 102 c, etc., that areoriented in a direction perpendicular to the orientation of acorresponding edge fragment. In addition, the simulation site 102includes a number of sample points 102 i, 102 j, 102 k, etc., that areoriented in a direction parallel with the edge fragment of the polygon.

A graph of the image intensity can be computed for the parallel andperpendicular sample points. For example, a graph 106 plots the changingimage intensity as the sample points 102 a, 102 b, 102 c get closertowards the edge of the polygon. A graph 108 plots the image intensityat the sample points along the edge fragment of the polygon. If theimage intensity along the edge fragment had little or no curvature, thegraph 108 should be relatively flat. However, if the graph 108 has acurve, the image intensity is likely not consistent along the length ofthe edge fragment. Therefore, in one embodiment of the invention,additional sample points 110 a, 110 b, 110 c, etc., and 112 a, 112 b,112 c, etc., can be added to the simulation site 102 if the imageintensity varies by more than a predetermined amount along the length ofthe simulation site. In one embodiment, the additional sample points110, 112, are oriented in a direction perpendicular to the length of theedge fragment. The image intensities can be calculated at each of thenew additional sample points 110, 112, and the information used tocalculate how the edge should be moved during OPC.

Once the placement of the simulation sites has been determined, oradditional simulation sites and/or sample points added, an expected edgeplacement error (EPE) is determined for the edge fragments. The EPE isused to determine how the edge fragment should be OPC corrected, if atall. If the edge fragment includes more than one simulation site, adecision must be made regarding which image intensity data should beused in correcting the position of the edge fragment during OPC. Forexample, in one embodiment, expected EPEs are calculated at eachsimulation site or along each set of sample points on the edge fragment.The maximum EPE is then used in the OPC correction of the edge fragment.Alternatively, the minimum EPE for the edge fragment could be used orthe average or some other mathematical combination of the EPEs could beused to determine how much, and in which direction, the edge fragmentshould be moved to improve image fidelity.

In yet another embodiment, the image intensity or EPE of an edgefragment may also be computed at each of the simulation sites/samplepoints assuming differing process conditions, such as illuminationintensity, illumination pattern, focus, polarization, partial coherencesettings, long range flare, etc. The image intensities or EPEs computedunder each of the different process conditions are used alone or incombination to determine the OPC correction and/or fragmentation of anedge fragment.

Although the disclosed embodiment of the invention calculates anexpected EPE for each simulation site and uses the EPE data to determinean OPC correction for an edge, it will be appreciated that it is notnecessary to calculate an EPE at each simulation site. Rather, the imageintensity data computed at each simulation site or set of sample pointscan be used alone or in combination to determine the OPC correction ofthe edge fragment. In addition, the adjustment of the simulation sitesand/or sample points may occur a single time or multiple times during anOPC correction process, such that each iteration adjusts the location ornumber of one or more simulation sites and/or the number of samplepoints.

Although the embodiments of the invention described above use thecalculated image intensity gradient vector to adjust the position of asimulation site, to add simulation sites to an edge fragment or to addsample points to simulation sites, it will be appreciated that thecalculated image intensity gradient vectors can also be used to adjustthe fragmentation of the polygons. For example, in areas where the imageintensity gradient vector indicates a curving image intensity,additional fragmentation end points may be added. Conversely, where thecalculated image intensity gradients indicate little curvature in theintensity gradient, fragmentation end points can be removed. In anotherembodiment, fragmentation end points can be added where the contour ofan estimated image intensity of a designated value such as that requiredto properly expose a wafer, crosses an edge fragment. This designatedvalue may be determined by a constant exposure threshold or calculatedusing a lithographic process model. The crossing points may bedetermined by interpolating the calculated image intensities that areestimated for neighboring simulation sites. Increasing the number offragmentation end points generally improves pattern fidelity by allowingfiner OPC adjustments but requires increased processing time. Removingfragmentation end points improves processing time at a cost of decreasedOPC resolution. These steps can be repeated iteratively to optimize eachstep of the OPC procedure as it executes.

After refragmentation, simulation sites are added to the newly creatededge fragments. In one embodiment, simulation sites are initially placedwith a rule such as placing the site at the center of each edge fragmentor according to the position of neighboring features, etc.

The initial placement can then be revised by calculation of the imageintensity gradient vectors at the simulation sites and repositioning thesimulation sites, adding more sites, or adding sample points to existingsimulation sites as described above. The process can be repeated in aniterative manner. Furthermore, simulation sites associated with the edgefragments that are unchanged may be adjusted as a result of adding orremoving fragmentation end points.

In yet another embodiment, the initial fragmentation and simulation siteselection can be based on simulations calculated on a fixed grid ofsample points regardless of the layout under consideration. For example,FIG. 6 shows a uniform grid 130 of sample points 132 at which estimatesof image intensity are calculated regardless of the position of afeature 134 in a layout. Alternatively, as shown in FIG. 7, the uniformgrid 130 may include a geometric filter 136 to eliminate sample points132 that are not near the boundaries of the feature 134.

Once image intensity estimates have been made at each of the samplepoints 132, the feature 134 is fragmented to form a series of edgefragments that are OPC corrected. The image intensity calculations atthe sample points 132 can determine the proper location of thefragmentation end points. Fragmentation end points can be placed atpositions that are the closest to a sample point 132 where the imageintensity has the desired value. Alternatively, the image intensityvalues can be interpolated to determine where the image intensitythreshold crosses an edge of the feature and therefore where thefragmentation end points should be located.

In some instances, the position of the one or more sample points 132associated with an edge fragment may be moved in accordance with animage intensity gradient vector as described above. One or more of thesample points 132 is associated with or mapped to each edge fragment forOPC purposes. The mapping may be made with a rule such as selecting theclosest sample points next to an edge fragment or selecting the samplepoint with the least desirable image intensity that is near the centerof the edge fragment. Alternatively, more complex algorithms may beused. The mapping of a sample point to an edge fragment may be static ordynamic during OPC iterations, etc.

FIG. 8 illustrates one possible computing environment for performing thepresent invention. A computer system 140 includes one or more processingunits that perform a set of instructions that are stored on a computerreadable media 142 or received embedded in a communication signal on acommunication link to perform the methods of the present invention. Aninitial layout is stored in a conventional file format such as GDS-II,or an equivalent, on a database 142, computer readable media such as aCD, DVD, tape drive, etc., or is received over a communication link. Thecomputer system 140 analyzes the layout to adjust the position of thesimulation sites and/or adjusts the number of simulation sites/samplepoints or fragmentation end points in order to produce OPC correctedlayout data in accordance with the embodiments of the invention asdescribed above. The OPC corrected data is stored in a memory, on acomputer readable media or in a database to be accessed by a maskwriting tool (not shown) in order to produce one or morephotolithographic masks or reticles used in a photolithographic process.

In an alternative embodiment of the invention, all or a portion of theinitial layout can be transmitted to a remote computer system 160 thatperforms the fragmentation and simulation site selection/modification orre-fragmentation in accordance with the present invention. The remotecomputer system 160 may be in the same country as the computer system140 or may be in a different country. The processed layout file or theOPC corrected layout data that is computed from the transmitted layoutfile is then transmitted to the computer system 140, or directly to themask writing tool, via a wired or wireless communication link 162, suchas the Internet, for use in creating photolithographic masks orreticles.

It will be appreciated that the relationship between fragmentationpoints and simulation sites and sample points can be complex. Thetechniques used in Matrix OPC, the subject of a previous U.S. patentapplication Ser. No. 10/387,224, hereby incorporated by reference, mayalso be applied to manage these relationships. While the disclosedembodiments have been primarily directed to performing OPC on the layoutdescription, it will be appreciated that the present invention is alsouseful with other resolution enhancement techniques including:generating phase-shifting mask layouts, compensating for off-axisillumination systems, compensating for polarization effects andtechniques for compensating for multiple exposures.

While the preferred embodiment of the invention has been illustrated anddescribed, it will be appreciated that various changes can be madetherein without departing from the scope of the invention. It istherefore intended that the scope of the invention be determined fromthe following claims and equivalents thereof.

1. A method of preparing a layout file defining features to be createdvia a photolithographic process for optical and process correction,comprising: reading at least a portion of a file defining one or morefeatures to be created; fragmenting the one or more features into anumber of edge fragments; defining an initial simulation site associatedwith a selected one of the edge fragments; using a computer, estimatingan image intensity at the initial simulation site; and adjusting theposition of the initial simulation site associated with the selected oneof the edge fragments based on the estimated image intensity, whereinthe adjusting comprises moving the position of the initial simulationsite to a new position on the selected one of the edge fragments.
 2. Themethod of claim 1, further comprising defining additional simulationsites associated with the selected one of the edge fragments based onthe estimated image intensity.
 3. The method of claim 1, wherein theinitial simulation site includes a number of sample points and whereinthe method further comprises adding additional sample points to theinitial simulation site.
 4. The method of claim 3, wherein the adding ofthe additional sample points is based on image intensities computed atthe sample points of the initial simulation site.
 5. The method of claim1, wherein the adjusting is based on an image intensity gradient vectorcomputed at the initial simulation site.
 6. The method of claim 5,wherein the adjusting comprises moving the position of the initialsimulation site by a distance that depends on an angle of the imageintensity gradient vector.
 7. The method of claim 5, further comprisingdetermining whether an angle of the image intensity gradient vectorexceeds a threshold, and wherein the adjusting of the position of theinitial simulation site is based on the determination.
 8. The method ofclaim 1, further comprising: computing an edge placement error at theinitial simulation site; and moving the selected one of the edgefragments based on the edge placement error.
 9. The method of claim 1,further comprising generating a mask from the prepared layout file. 10.The method of claim 9, further comprising fabricating an integratedcircuit using the prepared layout file.
 11. The method of claim 1,wherein the defining, estimating, and adjusting are performed for one ormore other initial simulation sites on other ones of the edge fragments.12. The method of claim 1, further comprising storing the preparedlayout file on a computer readable medium.
 13. A method of preparing alayout file defining features to be created via a photolithographicprocess for optical and process correction, comprising: reading at leasta portion of a file defining one or more features to be created;fragmenting the one or more features into a number of edge fragments;defining an initial simulation site associated with a selected one ofthe edge fragments; using a computer, estimating an image intensity atthe initial simulation site; adjusting the position of the initialsimulation site associated with the selected one of the edge fragmentsbased on the estimated image intensity; and adding additional edgefragments in accordance with an image intensity gradient vector computedat the initial simulation site.
 14. The method of claim 13, wherein theadditional edge fragments have fragmentation end points that arepositioned on a feature edge where a contour of an image intensity of adesignated value crosses the feature edge.
 15. A method of preparing alayout file defining features to be created via a photolithographicprocess for optical and process correction, comprising: reading at leasta portion of a file defining one or more features to be created;fragmenting the one or more features into a number of edge fragments;defining an initial simulation site associated with a selected one ofthe edge fragments; using a computer, estimating an image intensity atthe initial simulation site; adjusting the position of the initialsimulation site associated with the selected one of the edge fragmentsbased on the estimated image intensity; and reducing the number of edgefragments in accordance with an image intensity gradient vector computedat the initial simulation site.
 16. A computer readable storage deviceor memory on which is stored data regarding a layout for a number offeatures to be created via a photolithographic process, wherein the dataare created by: reading at least a portion of a file defining one ormore features to be created; fragmenting the one or more features into anumber of edge fragments; defining an initial simulation site associatedwith a selected one of the edge fragments; estimating an image intensityat the initial simulation site; and adjusting the position of theinitial simulation site associated with the selected one of the edgefragments based on the estimated image intensity, wherein the adjustingcomprises moving the position of the initial simulation site to a newposition on the selected one of the edge fragments.
 17. A method ofpreparing a layout data file for OPC, comprising: transmitting at leasta portion of a layout data file to a remote computer system thatprocesses the data file by: reading at least a portion of thetransmitted file defining one or more features to be created;fragmenting the one or more features into a number of edge fragments;defining an initial simulation site associated with a selected one ofthe edge fragments; estimating an image intensity at the initialsimulation site; and adjusting the position of the initial simulationsite associated with the selected one of the edge fragments based on theestimated image intensity, wherein the adjusting comprises moving theposition of the initial simulation site to a new position on theselected one of the edge fragments.
 18. A computer readable storagedevice or memory storing a sequence of instructions that when executedby a computer cause the computer to perform a method, the methodcomprising: reading at least a portion of a file defining one or morefeatures to be created; fragmenting the one or more features into anumber of edge fragments; defining an initial simulation site associatedwith a selected one of the edge fragments; estimating an image intensityat the initial simulation site; and adjusting the position of theinitial simulation site associated with the selected one of the edgefragments based on the estimated image intensity, wherein the adjustingcomprises moving the position of the initial simulation site to a newposition on the selected one of the edge fragments.
 19. The computerreadable storage device or memory of claim 18, wherein the methodfurther comprises defining additional simulation sites associated withthe selected one of the edge fragments based on the estimated imageintensity.
 20. The computer readable storage device or memory of claim18, wherein the initial simulation site includes a number of samplepoints and wherein the method further comprises adding additional samplepoints to the initial simulation site.
 21. The computer readable storagedevice or memory of claim 20, wherein the adding of the additionalsample points is based on image intensities computed at the samplepoints of the initial simulation site.
 22. The computer readable storagedevice or memory of claim 18, wherein the adjusting is based on an imageintensity gradient vector computed at the initial simulation site. 23.The computer readable storage device or memory of claim 22, wherein theadjusting comprises moving the position of the initial simulation siteby a distance that depends on an angle of the image intensity gradientvector.
 24. The computer readable storage device or memory of claim 22,wherein the method further comprises determining whether an angle of theimage intensity gradient vector exceeds a threshold, and wherein theadjusting of the position of the initial simulation site is based on thedetermination.
 25. The computer readable storage device or memory ofclaim 18, wherein the adjusting comprises moving the position of theinitial simulation site towards a point of greater image intensitycurvature.
 26. The computer readable storage device or memory of claim18, wherein the method further comprises: computing an edge placementerror at the initial simulation site; and moving the selected one of theedge fragments based on the edge placement error.
 27. The computerreadable storage device or memory of claim 18, wherein the methodfurther comprises performing the acts of defining, estimating, andadjusting for one or more other initial simulation sites on other onesof the edge fragments.
 28. A computer readable storage device or memorystoring a sequence of instructions that when executed by a computercause the computer to perform a method, the method comprising: readingat least a portion of a file defining one or more features to becreated; fragmenting the one or more features into a number of edgefragments; defining an initial simulation site associated with aselected one of the edge fragments; estimating an image intensity at theinitial simulation site; adjusting the position of the initialsimulation site associated with the selected one of the edge fragmentsbased on the estimated image intensity, wherein the adjusting is basedon an image intensity gradient vector computed at the initial simulationsite; and adding additional edge fragments in accordance with the imageintensity gradient vector.
 29. A computer readable storage device ormemory storing a sequence of instructions that when executed by acomputer cause the computer to perform a method, the method comprising:reading at least a portion of a file defining one or more features to becreated; fragmenting the one or more features into a number of edgefragments; defining an initial simulation site associated with aselected one of the edge fragments; estimating an image intensity at theinitial simulation site; adjusting the position of the initialsimulation site associated with the selected one of the edge fragmentsbased on the estimated image intensity, wherein the adjusting is basedon an image intensity gradient vector computed at the initial simulationsite; and reducing the number of edge fragments in accordance with theimage intensity gradient vector.